Investigating Micellar Phases in Hydrated Nonionic Surfactants

Some surfactants – amphiphilic molecules that comprise a hydrophilic headgroup and hydrophobic tail – form lyotropic liquid crystal (LLC) phases upon the addition of water. LLCs have applications that range from water desalination membranes and drug delivery to templates for inorganic materials. To effectively design surfactant-water systems for useful applications, we must better understand the detailed molecular features that govern their LLC phase behaviors. While complex micellar phases have been reported in both block polymer and ionic surfactant systems, previous studies of the phase behaviors of aqueous LLCs of non-ionic surfactants lack definitive phase assignments. However, nonionic polyoxyethylene alkyl ether (C_iE_j) surfactants do demonstrate the potential for complex micellar phase formation. Thus, we investigated the ordered aqueous micellar phases of a C_iE_j surfactant using high-resolution small-angle X-ray scattering as functions of varying hydration and temperature. We report the formation of BCC and FCC phases, and most significantly, the observation of Frank-Kasper A15 and σ phases. From our results, we conclude that nonionic surfactants indeed form complex liquid crystal phases that bridge the gap between polymer and ionic surfactant systems.